The properties of a single-walled carbon nanotube strongly depend on its geometric structure. This structure is often characterized by identification and enumeration of chiral indices (n,m). The integers n and m denote the number of unit vectors along two directions in the honeycomb crystal lattice of graphene. These indices can be used to determine important parameters of a nanotube, such as whether the nanotube is metallic (n=m), semimetallic (n−m is a multiple of 3), or semiconducting (other values of n−m). Nanotubes having a value of m=0 are generally referred to as zigzag nanotubes, and nanotubes having values of n=m are generally referred to as armchair nanotubes. Nanotubes having values of n and/or m differing from zigzag and armchair nanotubes are generally referred to as being chiral nanotubes.
A wide variety of growth techniques have been developed to make single-walled carbon nanotubes. Each technique generally produces a population of nanotubes having a distribution of (n,m) indices. While a number of these techniques have focused on providing bulk samples of SWCNTs being enriched in one or more preselected chiralities, most or all prior methods present a number of deficiencies. For example, the distribution of chiralities in a bulk sample often depends on growth conditions and thus can show large variations even within the same method. Further, some prior methods have been unable to provide high ratios of a desired chirality in a bulk sample. In order to remedy these deficiencies, a number of techniques have been devised to purify SWCNT samples by removing SWCNTs having one chirality or another in a post-growth step. Unfortunately, such approaches generally increase process time, increase process costs, and/or result in a relatively low yield of nanotubes of the desired chirality relative to the size of the unpurified bulk sample. Further, such purification techniques may be incapable of providing a bulk sample enriched in a small number of desired chiralities, such as a bulk sample enriched in as few as one or two desired chiralities. Thus, there is a need for improved methods for making and/or purifying semiconducting single-walled carbon nanotubes.